There are a variety of methods available for creating a hollow shell. In many cases, for example with many outdoor balls, two halves are made independently and then are joined together along a seam. Such a process of making a shell may allow for one half to be made very inexpensively with no added features and only the other half needs to have any sort of specialized additions, such as a valve to allow air in, where the ball is designed to be inflated. In other examples, such as a traditional hard plastic ball or wiffle ball, the two halves are joined along a seam and the two halves may be identical.
However, it is not always desirable to have a seam on a shell. In such an event, there are several conventional methods of making a single piece hollow shell.
In a first embodiment, a wax or other meltable structure is used on the inside of the mold. The wax structure is placed in the mold and the material to make the shell is poured in around the wax. A perforation or hole remains in the shell after molding, so that a heating implement can be inserted into the wax to cause it to melt and drain from the shell. In using such a system, the mold temperature necessary to mold the shell of the desired material is considered to determine whether the use of wax or another temporary material to create the mold inner surface is feasible. Alternative ways of doing a similar method include using a fluid filled bladder, where the fluid is drained after molding, and the insertion of a cannula that blows air at a sufficient force to force the molding material away from the end of the cannula and towards the mold walls to create a cavity.
In other instances, a blow molding process may be used to create a hollow article. Such a system again has limited application, as only certain types of resins are compatible with blow molding. In addition, if the desired final shape has a plurality of holes, blow molding may create a less optimal result, as it is typically used when a continuous surface is desired.
The use of a perforated single-piece hollow shell structure is particularly desirable in the context of a golf ball. In some instances, the use of two halves of a material being joined along a seam is not desirable. Regardless of the joining method, the use of a seam can create a discontinuity within the ball. This discontinuity can create additional stresses to the ball on impact. In other cases, it can negatively affect the flight path of the ball when it is struck at particular angles relative to the seam. The use of a perforated layer in a golf ball can help in creating a golf ball with multiple integrally-formed layers that can be secured to one another mechanically, rather than chemically, thereby improving the recyclability of the inner layers of the ball.
The previously developed structures and methods do not provide a method of making a single-piece hollow shell with a plurality of perforations therethrough. In addition, such methods may limit the resins available for forming the hollow shell. Accordingly, a method of forming a hollow shell with perforations therethrough that may be made with a variety of resins is desirable. It is also desirable to develop a golf ball incorporating such a hollow structure.